Demo: The write System Call

A program running in user mode cannot touch hardware, files or other processes on its own. Whenever it needs the operating system to do something privileged it asks the kernel by performing a system call. Normally libc issues that system call on your behalf, hidden inside printf() or write(); in this demo there is no libc below us at all.

Together with the teaching assistant you will fill in the TODOs in main.c, building three layers on top of each other — my_puts() over my_write() over the raw my_syscall() — then run the result under strace to confirm the bytes really did reach the kernel through the syscall instruction and nothing else.

The my_syscall() function is given to you, and you will reuse it unchanged in every exercise of this session.